Apparatus for color control of objects

ABSTRACT

An apparatus for color control of objects (11) has an approximately point-shaped light source in the form of a xenon flash lamp (16), which illuminates a specific, desired area through a diaphragm (17). Light reflected from the object is received by a detector unit (13) having three or more sensors (22) with their respective spectral sensitivities, which are positioned at such a great distance from the object that each receives substantially the same amount of light from all parts of the illuminated area. The detector signals are amplified by amplifiers adapted to filter all signals exhibiting another timewise variation than the light source, so that disturbing influence from the surroundings is avoided. The apparatus is simple and fast and is versatile in use.

The invention relates to an apparatus for colour control of an object,the apparatus containing a light source for illuminating the object anda detector for filtering and measuring the light reflected from theobject.

In industrial colour control or colour sorting it is often necessary tocontrol objects which move with a great velocity, have different sizes,are hot or wet or are difficult being into contact with colour controlequipment for other reasons. It is therefore desirable that the coloursof the objects can be controlled at a distance.

It is known, e.g. from German Patent DE 3 244 286 to perform colourcontrol by illuminating an object with light from a pulsed source oflight, whereupon the reflected light is detected. The reflected light isimaged through a lens system on a sensors of the detector unit. Sincethe light source is pulsed, light having another timewise variation maybe filtered by suitable signal processing. The detected light is thencompared with a reference signal from the pulsed object source, thecolour of the light is determined.

CH 612.760 relates to an optical system, where an object is illuminatedby obliquely incident light, and where the reflected light is collectedby a lens system and passed to sensors through an optical fibre system.

It is a problem in colour measurement that the colour detectors have afinite light-sensitive area. The sensitivity in this area is constantfor an ideal sensor, but in practice the sensitivity of the sensors canvary very strongly even within small distances of the light-sensitivearea. This gives problems when an object point is imaged in an imagepoint. Thus, the detected light intensity will depend upon the positionof the object, where even small changes in the position of the objectwill entail that different light intensities are measured. Since knowncolour measuring devices, where the measurement takes place withoutcontact between the object and the detector, mainly image the object ona plurality of sensors, it will be impossible to obtain a constantcolour measurement when an object, e.g. a shrimp, passes through themeasurement area on a conveyor belt, or if the measurement area is verylarge, e.g. 1 m². When imaging is performed on the sensors, anon-controllable, weighted averaging of the colour of the measurementarea is effected. In addition, a lens system is to be trimmed tominimize the chromatic aberrations. To minimize these aberrations it isknown to use an aperture diaphragm so that the used part of the lensesis limited, thus reducing the light intensity. For the measurement areato be imaged on, e.g. three detectors, the system must be strictlysymmetrical.

Another problem associated with prior art apparatuses is that it is notpossible to perform colour measurement where the object and thedetector/light source are separated by a great distance. Thus, the closeproximity of the detection/light source to the object in prior artapparatuses poses a cleaning problem when the colour of wet or dustyobjects, e.g., fish, shell fish, chips, egg white, etc., on a conveyorbelt is to be measured. Additionally, objects having differentthicknesses pose problems if their mutual thickness variation is notsmall with respect to the measurement distance. Such a variation causesproblems. In particular, in a system where the measurement is imaged onthe sensors.

For objects to be conveyed with an even speed it is necessary to performsimultaneous colour measurements.

In the prior art it is often a problem that the useful wavelength rangeis small. This band limitation is caused by the fact that the knowncolour measuring apparatuses contain different sophisticated componentswhose losses are wavelength dependent, e.g. imaging components.Frequently, it may be advantageous to have a measurement range from 300nm (ultra violet) to 2000 nm (infra red) at disposal.

Therefore, the object of the invention is to provide an apparatus whichis capable of giving reproducible measurements of a colour of an object,irrespective of the objects' position in the measurement area. Further,the measurement must be contact-free and the representation of thecolour of the object must be independent of variations in thickness.

Another object of the invention is to provide an apparatus whichminimizes the number of optical components used. A further object of theinvention is to provide a great measurement distance in order to varythe dimensions of the measurement area, advantageously from above 1 m²to below 1 mm². Yet another object of the invention is to provide anapparatus capable of making colour measurements far beyond the range ofthe visible spectrum.

These objects are achieved by an apparatus for measuring the averagevalue of colour in a measurement area of an object, the apparatuscomprising a light source for illuminating the object, the light sourcebeing adapted to illuminate the measurement area evenly and with greatintensity, the measurement area comprising the part of the object thecolour of which is to be measured; means for defining the measurementarea, the means being interposed between the light source and theobject; and a detector for measuring light reflected from the object,the detector including a plurality of sensors, each sensor having afield of vision containing the entire measurement area; wherein (i) thedetector is located at a position such that each sensor image-freereceives substantially the same amount of light from all parts of themeasurement area and (ii) the average distance between the detector andall parts of the measurement area is greater than the variation indistance therebetween.

Means for defining the measurement area is interposed between the lightsource and the object and is only used for varying the size of theilluminated area. Such means includes, for example, a lens and adiaphragm. The diaphragm is a visual field diaphragm in contrast to theaperture diaphragms which are used in the prior art to improve the imagequality. The colour then recorded by the sensors will thus be indicativeof the average colour in the entire illuminated area. Since no imaginglens systems or the like are incorporated between the object and thedetector, the apparatus is not sensitive to changes in the position ofthe object within the measurement area. Since the number of opticalboundary faces is limited, the losses in the system are likewiselimited. There are no substantial propagation losses between object anddetector, which makes it possible to measure with a great distancebetween object and measuring equipment.

Objects having various sizes and irregular shapes (e.g., shrimp) can bemeasured when the objects are placed on a black background or when lightfalling outside the objects is otherwise prevented from reaching thedetectors. These measurements will be reproducible and independent ofwhere the objects are present in a measurement area, because all partsof the measurement area provide equal contributions to the colourrepresentation formed by the detector.

The measurement area can be formed in any manner (e.g., by illuminatingan object on a conveyor belt in a streak transverse to the conveyorbelt), and the size of the object can be varied within a very largerange. Since the size of the measurement area is adjusted solely byregulating the aperture of the light source, a simple and inexpensiveoptical system is obtained, where changing to new objects is performedmerely by changing the aperture of the light source. Thus, in the caseof visual inspection, it is easy to change the size of the measurementarea while simultaneously checking whether the measurement area hasmoved, because the measurement area corresponds to the illuminatedregion.

Further, the apparatus is suitable for measuring an average value forobjects having greatly varying colour patterns.

When the light source is a xenon flash lamp, means for defining themeasurement area advantageously comprise a convex lens and a diaphragmwhich are interposed between the light source and the object to beilluminated such that the illumination can be limited to just thedesired measurement area which may have an arbitrary, desirable form.

Advantageously, the apparatus according to the invention includes meansfor filtering signal contributions from the surroundings. In a preferredembodiment, the detector contains amplifiers for amplification ofelectric signals, each of which signals represents the light passedthrough a colour selective element, which amplifiers are adapted tofilter the signal contributions originating from light having atime-wise variation which is different than that of the illuminationpulses.

In another embodiment, the invention is directed to a method formeasuring a dimension of an object in a specific direction which methodcomprises illuminating the object in a streak in the direction andmeasuring the light reflected from the object with an apparatusaccording to the invention. The reflected light intensity measured bythe detector of the apparatus is a measure of the dimension, e.g.,width, of the object in the beam direction.

If the dimensional measurement in accordance with the invention isperformed in a plurality of specific positions, the shape of the objectcan be determined. Thus, in another embodiment, the invention isdirected to a method for determining the shape of an object. Inaccordance with this embodiment, the object can be positioned, forexample, on a conveyor belt and the individual dimension measurementscan be performed in predetermined positions on the conveyor belt.

In yet another embodiment of the invention, the apparatus includes meansfor eliminating light contributions from a specular surface of an objectwhose colour is to be determined. Means for eliminating lightcontributions from a specular surface comprises, for example, a firstpolarization filter having a first polarization direction and a secondpolarization filter having a second polarization direction, the firstpolarization filter being interposed between the light source and theobject and the second polarization filter being interposed between theobject and the detector. This arrangement of filters is capable ofeliminating light contributions from a specular background because it isknown that polarized light remains polarized after specular reflection,but is spread as diffuse light by a non-specular surface.

An embodiment of the apparatus of the invention is shown schematicallyin the drawing and will be described in more detail below with referenceto this drawing.

In the drawing, 10 is a conveyor belt on which objects 11 to be colourcontrolled are conveyed. When an object arrives at the control positionat an illumination unit 12 and a detector unit 13, a control unit 14 isinformed that measurement is to take place. This information is passedon to the detector unit 13 and via this to the illumination unit 12. Thelatter has a power supply unit 15 with a trigger to trigger a xenonflash lamp 16, which forms an approximately point-shaped light sourceand emits a strong light pulse with a duration of 2-6 μs on reception ofthe signal from the control unit. This light passes through a visualfield diaphragm 17 having a shape corresponding to the area of theobject 11 whose colour is to be controlled and through a convex lens 18which sends a bundle of substantially parallel rays 19 towards theobject 11.

When the object is non-specular, it spreads diffuse light, and part ofthis light impinges on the detector unit 13 which has a lens 21 whichsends the light towards three sensors 22 having different spectralsensitivities. The detector unit is positioned at such a great distancefrom the object that each sensor receives substantially the same amountof light from the various parts of the illuminated area from the object.The lens 21 may be omitted since it only serves as an amplifying elementand does not serve as an imaging element. The detector does not satisfythe imaging equation, but is positioned considerably more closely to thelens than to the image point. The sensors may optionally each have theirown colour filter (not shown). The signals from the sensors areamplified by amplifiers 23, which are adapted to only allow signalshaving the same timewise variation as the light source to pass, and theamplified analog signals are converted to digital signals in an A/Dconverter and a computing unit 24. The computing unit computes thecolour and intensity of the light on the basis of the strength of thethree signals, and the measurement result is compared with stored valuescorresponding to approved/rejected for the object in question. Whensorting, the measurement results is compared with a plurality of storedlimits, and the object is classified accordingly.

The object may also be fluorescent and be illuminated with excitationlight with a specific wavelength through a colour filter.

The apparatus may be used for size determination by illumination of anarea having at least the same size as the object. The measured intensityof light with the colour of the object is indicative of the size (area)of the object. If the object is placed on a background having a colourdifferent from the object, the reliability of the size determinationwill be particularly great, it being possible to determine the intensityof light both with the colour of the object and with the colour of thebackground. This procedure is of particular importance if the colour isnot quite constant from object to object.

When applying the system to width determination, The object isilluminated in a narrow streak transversely to the object. The measuredintensity of light having the colour of the object is indicative of thewidth of the object.

Shape determination may be performed by positioning the object on aconveyor belt running past a width determining set-up as describedabove. Comparison of measurements of the width of the object withsimultaneous measurements of the position of the conveyor belt providesan indication of the shape of the object. It may e.g. be determined inthis way whether an ice-cream cone has its tip pointing forwardly orrearwardly on the conveyor belt, which is important in automaticpackaging procedures.

When using a set of intersecting polarization filters 25, 26, placed atthe illumination units and at the detector unit, respectively, theapparatus may be used for colour determination of objects present on aspecular background, since specular light is completely suppressed bythe polarization, while only 75% of diffuse light disappears. Similarly,undesirable reflections from wet glossy or otherwise specular surfacesare suppressed.

This set-up may also be used for detecting whether an impurity ispresent on a specular surface and for determining the colour of theimpurity. What is in mind here is particular detection whether an eggwhite present in a stainless steel container is polluted with yolk orsomething else. When using the colour controlling properties of theapparatus it may be determined whether the impurity consists of yolk,blood or yolk string.

The shown and described apparatus may be modified in several ways withinthe scope of the invention. E.g., a modulated lamp, which may be a xenonlamp, may be used as a light source instead of a flash lamp.

I claim:
 1. An apparatus for measuring the average value of colour in ameasurement area of an object, said apparatus comprising:(a) a lightsource for illuminating said object, said light source being adapted toilluminate said measurement area evenly and with great intensity, saidmeasurement area comprising the part of the object the colour of whichis to be measured; (b) means for defining the measurement area, saidmeans being interposed between said light source and said measurementarea; and (c) a detector for measuring light reflected from said object,said detector including a plurality of sensors, each sensor having afield of vision containing the entire measurement area; wherein (i) thedetector is located at a position such that each sensor image-freereceives substantially the same amount of light from all parts of themeasurement area and (ii) the average distance between the detector andall parts of the measurement area is substantially greater than thedistance between the individual sensors.
 2. An apparatus according toclaim 1, wherein said light source is a xenon flash lamp and said meansfor defining said measurement area includes a convex lens and adiaphragm interposed between said xenon flash lamp and said object, saidconvex lens and said diaphragm limiting the illumination of the objectso as to define the desired measurement area.
 3. An apparatus accordingto claim 1, wherein said light source is illuminating the measurementarea with pulses of short duration, said detector further includesfilter means which filters out signal contributions originating fromlight having a time-wise variation which differs from the time-wisevariation of light from the light source.
 4. An apparatus according toclaim 3, wherein said detector comprises amplifiers for amplification ofsignals from the sensors, said signals representing light passed througha colour selective element, said amplifiers filtering out signalcontributions originating from light having a time-wise variation whichdiffers from the time-wise variation of light from the light source andalso which amplifies signals having the same time-wise variation as thatof the light from the light source.
 5. An apparatus according to claim1, wherein said measurement area is partly specular and partlydiffusing, and further including means for eliminating lightcontributions from said specular surface, said means being placed in theoptical path between said light source and said detector.
 6. Anapparatus according to claim 5, wherein said means for eliminating lightcontributions from said specular surface comprises a first polarizationfilter having a first polarization direction and a second polarizationfilter having a second polarization direction, said first polarizationfilter being interposed between said light source and said object andsaid second polarization filter being interposed between said object andsaid detector.